Casing vent flow/gas migration (CVF/GM) analysis is becoming a major concern for oil/gas producers around the world. In order for the gas to negotiate itself from the source to surface, a path must be present. This path can be due to fractures around the wellbore, fractures in the production tubing, poor casing to cement/cement to formation bond, channeling in the cement, or various other reasons.
Well logging is performed at various stages in the life of a well—during the drilling process (pre-production), while a well is in operation (production) and periodically when the well is no longer in service (abandoned). Information obtained by well logging may include temperature, pressure or acoustic information on the wellbore, production tubing, surrounding casing or reservoir matrix, geological makeup of the strata through which the wellbore is drilled, or the reservoir matrix, and the like.
Methods currently used in the oil and gas industry for well logging include, for example, Pulsed Neutron Neutron logging (PNN) (used for assessing the elements in a formation), Cement Bond Logging (CBL) (used for assessing casing cement integrity), noise/temperature logging, Radial Bond Logging (RBL), Compensated Neutron Logging (CNL) (used for assessing porosity of a formation). Seismic detection methods using geophones and artificial acoustic signal sources, provide information relating to the geologic strata in the area of the well. For example, acoustic sensing systems employing optical sensors and fiber for downhole seismic applications are known. CA2320394 describes a system for detecting an acoustic signal produced by an artificial source in a second wellbore to identify differential propagation of acoustic waves in the earth formation. CA 2342611 discloses a system including an acoustic transmitter (an artificial source) for seismic sensing, for use in acquiring information about the properties of the earth formations in the borehole where it is deployed. Artificial sources for the acoustic signal may be used, such as an air gun, a vibrator, an explosive charge or the like to produce a seismic wave. These may be quite violent, producing an acoustic signal that is felt on the surface, or at a significant distance from the source.
CVF/GM may occur at any time in the life of the well. Wells found to have aberrant or undesired fluid (generally, gas or liquid hydrocarbon) migration (a ‘leak’) must be repaired to stop the leak. This may entail halting a producing well, or making the repairs on an abandoned or suspended well. The repair of these situations does not generate revenue for the gas company, and can cost millions of dollars per well to fix the problem.
In order to deal with the leak, a basic strategy may include these steps: identify the gas source that is responsible for the problem; communicate with the leaking fluid source (i.e. making holes in production tubing and/or cement in order to effectively access the formation), and; plug, cover or otherwise stop the leak (i.e. inject or apply cement above and into the culprit formation in order to seal or ‘plug’ the gas source, preventing future leaks).
Materials and Methods for stopping leaks associated with oil or gas wells are known, and usually involve injection of a liquid or semiliquid matrix that sets into a gas-impermeable layer. For example, U.S. Pat. No. 5,500,3227 to Saponja et al. describes methods of terminating undesirable gas or liquid hydrocarbon migration in wells. U.S. Pat. No. 5,327,969 to Sabins et al describes methods of preventing gas or liquid hydrocarbon migration during the primary well cementing stage.
Before the leak can be stopped however, it must be identified and localized. Existing systems for identification of a leak comprise a detection device, such as a single microphone at the end of a cable or wire. The microphone is lowered into the well, and suspended at a depth of interest, and background acoustic activity at that depth is recorded for a short period of time. The device is then raised up a short distance (repositioned) and the process repeated. The recording interval may range from about 10 seconds to about 1 minute, and the repositioning distance from about 2 meters to about 5 meters. Longer recording intervals and shorter repositioning distances may give more accurate data, but at the expense of time. Once data collection is complete, the acoustic data is processed and the noise signature of the well characterized. This serial, stepwise monitoring of well depths is slow—a typical well may take 6-12 hours to log. For deep wells, the time involved in this serial data acquisition can be substantial. For example, total logging time, comprising stabilization time, repositioning and actual recording time for each depth may take up to 12 hours for a 1000 m well. Additionally, as the recording device is only recording data at each depth for one minute or thereabouts, the recording device may not be directly at the leak point when a noise anomaly occurs—for a well with a low leak rate, a noise anomaly may be missed altogether. The length of the wire, and in the case of an analog signal, filtering and bandwidth limitations, also take a toll on the data by the time it is actually received uphole into the computer acquisition system, resulting in a poor signal to noise ratio.
Acquisition of reliable data in a timely manner for identification of the gas source is a key step in the process of stopping leaks from a wellbore, and improved methodologies and apparatus are desirable.